The invention relates to a process for treating waste water produced in the forest industry, in which process the waste water is further treated with chemicals after a biological treatment.
The waste waters of the forest industry contain varying amounts of dissolved and solid organic and inorganic substances derived from the raw materials used. The waste waters thus pollute the watercourses into which they are discharged. Solid insoluble substances make the water turbid and form a layer of sludge on the floor of the watercourse. Organic substances, regardless of whether they are dissolved or solid, consume oxygen from the water and directly or indirectly give it a taste or odor.
In spite of the fact that in the past 10-20 years the production of pulp has multiplied in Finland, the waste water load caused by the forest industry has been substantially reduced. The solution to the problem of waste water has, especially in Finland, been the activated sludge process, in which the organic substance is decomposed by micro-organisms. By the activated sludge process, as well as by other biological processes, it is possible to remove from waste water primarily small-molecular organic matter which causes biological oxygen demand (BOD).
The licence conditions set by the authorities presuppose, for reasons of environmental protection, also the removal of solids from waste waters. Solids have indeed been removed by using combinations of biological and chemical processes.
When the activated sludge process is used, the reduction of matter causing chemical oxygen demand (COD), as well as of nutrients such as phosphorus and nitrogen, remains considerably lower than the reduction of matter causing biological oxygen demand. Nutrients are derived not only from waste waters themselves but also from the chemicals possibly added at the biological treatment stage.
It is known to remove large-molecular organic matter (COD) and phosphorus from waste waters by means of chemical precipitation, for example by using aluminum or iron coagulants.
DE application publication 2 521 893 (SCA Development AB) proposes the removal of color and the matter which causes chemical oxygen demand (COD), present in the waste waters from pulp production, especially in waste liquors, by means of hydrogen peroxide and a ferrous salt. However, a satisfactory removal of the matter causing biological oxygen demand is generally not achieved by the use this method.
A process based on oxidation for the treatment of waste waters is also otherwise known. The oxidation of waste water is carried out by using hydrogen peroxide, the oxidative action of which can be enhanced by means of a catalyst, for example, a metal ion, UV light, or ozone.
The latest licence conditions for the waste waters of the forest industry set high requirements on emissions; not only the removal of matter causing chemical oxygen demand (COD) and of phosphorus but also the removal of organic chlorine compounds (AOX) is required. Organic chlorine compounds are formed in connection with bleaching with chlorine chemicals. Since nitrogen, in the form of both organic and inorganic, dissolved, insoluble and colloidal compounds, is also present in the waste waters, the reduction of nitrogen emissions has been set as an objective in licence conditions.